Oxfam formally apologized to Haiti Monday over the prostitution scandal rocking the aid charity, expressing its "shame" and vowing to do better as it handed over a damning internal report into the allegations.

Made public earlier in the day, Oxfam's 2011 report into the behavior of aid workers sent to Haiti following a devastating earthquake revealed that a former top official admitted to paying for sex and that three staff physically threatened a witness.

"We came here to share the report with the minister and express our shame and apologies to the Haitian government and to the Haitian people," said Simon Ticehurst, Oxfam's regional director for Latin America and the Caribbean.

"We've taken lots of measures to improve internal safeguarding measures," he said following a more than two-hour meeting with Haiti's minister of planning and external cooperation, Aviol Fleurant, who had summoned the charity to explain itself.

Oxfam's 2011 report, compiled in the year after aid workers were deployed to Haiti, revealed that seven staff were accused of using prostitutes at an Oxfam-funded residence.

Country director Roland Van Hauwermeiren admitted paying for sex and was offered a "phased and dignified exit" of resignation if he cooperated with the inquiry.

'Physical' threats

The report also said three Oxfam employees were involved in "physically threatening and intimidating" a witness who spoke to the investigators.

Four staff were fired for gross misconduct and three others, including Van Hauwermeiren, were allowed to quit.

Oxfam isn't out of the woods yet, reports Benedicte Paviot in London

Details of the Haiti scandal surfaced earlier this month and have engulfed Oxfam, drawing widespread condemnation and putting its funding at risk.

British Prime Minister Theresa May on Monday described the matter as "horrific," adding that "it was far below the standards that we expect for the charities and the NGOs that we're working with."

"We will not work with anybody who does not meet the high standards that we set and we believe are important," May added during a visit to a London school.

The charity has been suspended from bidding for new government funding until it undertakes reforms.

Oxfam has denied trying to cover up the allegations but admitted it could have been more open at the time, saying it was publishing the report "in recognition of the breach of trust that has been caused."

The Haitian government has expressed outrage and launched its own inquiry.

Haitian President Jovenel Moise tweeted last week that there was "nothing more unworthy or dishonest than a sexual predator who uses his position as part of the humanitarian response to a natural disaster to exploit needy people in their moment of great vulnerability."